TUESDAY, May 10, 2016 (HealthDay News) -- Two cases of children who developed iodine deficiency highlight the risks of putting too many restrictions on young kids' diets, researchers say.

The doctors said that the children -- aged 2 and 5 -- developed iodine deficiency because their diets lacked salt, dairy products, bread and other sources of the mineral.

Iodine deficiency is common in developing countries, but was virtually eliminated in the United States after iodized salt was introduced almost a century ago.

However, it can still happen if a child's diet is strictly limited, said study author Dr. Brigid Gregg. She is a pediatric endocrinologist at the University of Michigan's C.S. Mott Children's Hospital.

"Parents should be aware that if they're really restricting the foods their children eat, iodine deficiency is a possibility," Gregg said.

The body needs iodine to make thyroid hormones, which help control metabolism and other essential functions. Lacking iodine, the thyroid gland can become underactive -- what's known as hypothyroidism.

In infants and young children, that can result in stunted growth and even brain damage, Gregg pointed out.

Still, she said, when a young child is not growing well, hypothyroidism is "not the first thing that jumps to mind." And when hypothyroidism is suspected, doctors would not immediately think iodine deficiency is at the root of the problem, she added.

"We don't typically think of it in this country, because we've considered the problem 'solved' in the U.S.," Gregg said.

One of the children her team treated was 5 years old and had been diagnosed with autism. His parents were keeping him on a gluten-free, casein-free diet -- which meant he wasn't getting the dairy products and grains that are major iodine sources in the typical American diet.

From there, the boy was tested for autoimmune thyroid disease, in which the immune system mistakenly attacks the gland. But there was no evidence that his immune system had turned against him.

Instead, it turned out to be iodine deficiency.

The second case involved a 2-year-old boy with stunted growth and a limited diet -- both because he refused many foods, and because of his parents' restrictions. They'd replaced cow's milk with coconut milk, avoided processed foods and used sea salt instead of table salt.

Those were well-intended moves, Gregg said. The problem was, the child's diet had no good iodine sources.

Neither sea salt nor coconut milk contain the mineral, said Marina Chaparro, a clinical dietitian and spokesperson for the Academy of Nutrition and Dietetics.

"I'd recommend using iodized salt," said Chaparro, who was not involved in the study. "That's probably the easiest way to make sure your child is getting enough iodine."

And it doesn't take a lot, she noted: A half teaspoon of iodized salt each day would be enough for a child.

Milk, yogurt and commercially made breads are probably the most child-friendly food sources of iodine, according to Chaparro. But fish and seaweed also contain it, she said.

To Chaparro, the two cases highlight a broader issue: Children with restricted diets could be at risk of various nutritional deficiencies if parents aren't well-informed.

"Children aren't small adults, and we need to be mindful about placing our own dietary restrictions on them," Chaparro said.

She suggested that when parents do eliminate certain foods -- such as meat or dairy or gluten-containing grains -- they talk to their pediatrician or a dietitian about how to best replace those foods.

Chaparro also cautioned parents against getting caught up in diet trends. "Never put a child on a fad diet," she said.